PBX: Handle errors from AST_VECTOR_APPEND.

This resolves potentials leaks on AST_VECTOR_APPEND error in:
* ast_context_add_include2
* ast_context_add_switch2
* ast_context_add_ignorepat2

Change-Id: Ib60e95c4f622fa3b832d87227c0523a695d736b6